When an image forming apparatus which forms an image by an electrophotographic process adopts a direct transfer system of transferring an image by bringing a transfer member into contact with a photoconductor, the transfer member uses a conductive rubber roller having a conductive rotating shaft. Driving of the transfer member is controlled to match the process speed of the photoconductor.
A voltage applied to the transfer member is a DC bias voltage. At this time, the polarity of the DC bias voltage is identical to that of a transfer voltage for general corona discharge. To achieve satisfactory transfer using the transfer roller, a voltage of generally 3 kV or more (required current is several μA) must be applied to the transfer roller. This high voltage necessary for the above image forming process is conventionally generated using a wire-wound electromagnetic transformer. The electromagnetic transformer is made up of a copper wire, bobbin, and core. When the electromagnetic transformer is used in application of a voltage of 3 kV or more, the leakage current must be minimized at each portion because the output current value is as small as several μA. For this purpose, the windings of the transformer must be molded with an insulator, and the transformer must be made large in comparison with supply power. This inhibits downsizing and weight reduction of a high-voltage power supply apparatus.
In order to compensate for these drawbacks, it is examined to generate a high voltage by using a flat, light-weight, high-output piezoelectric transformer. By using a piezoelectric transformer formed from ceramic, the piezoelectric transformer can generate a high voltage at higher efficiency than that of the electromagnetic transformer. Since electrodes on the primary and secondary sides can be spaced apart from each other regardless of coupling between the primary and secondary sides, no special molding is necessary for insulation. The piezoelectric transformer brings an advantage of making a high-voltage generation apparatus compact and lightweight.
An example of the high-voltage generation apparatus by using the piezoelectric transformer is disclosed in, e.g., Japanese Patent Application Laid-Open No. 11-206113.
A conventional high-voltage power supply circuit using a piezoelectric transformer will be explained with reference to FIG. 6. In FIG. 6, reference numeral 101Y denotes a piezoelectric transformer (piezoelectric ceramic transformer) for a high-voltage power supply. An output from the piezoelectric transformer 101Y is rectified and smoothed to a positive voltage by diodes 102Y and 103Y and a high-voltage capacitor 104Y, and supplied to a transfer roller (not shown) serving as a load. The output voltage is divided by resistors 105Y, 106Y, and 107Y, and input to the inverting input terminal (negative terminal) of an operational amplifier 109Y via a protection resistor 108Y. The non-inverting input terminal (positive terminal) of the operational amplifier receives, via a resistor 114Y, a high-voltage power supply control signal (Vcont) which serves as an analog signal and is input to a connection terminal 118Y from a DC controller 201. The operational amplifier 109Y, the resistor 114Y, and a capacitor 113Y construct an integrating circuit. The control signal (Vcont) smoothed by an integral time constant determined by the component constants of the resistor and capacitor is input to the operational amplifier 109Y. The output terminal of the operational amplifier 109Y is connected to a voltage-controlled oscillator (VCO) 110Y. A transistor 111Y whose output terminal is connected to an inductor 112Y is driven to supply power to the primary side of the piezoelectric transformer.
The high-voltage power supply unit of an electrophotographic image forming apparatus comprises a plurality of high-voltage power supply circuits (corresponding to image forming units for, e.g., yellow (Y), magenta (M), cyan (C), and black (K)) using the piezoelectric transformer shown in FIG. 6. The high-voltage power supply unit outputs biases for charging, development, transfer, and the like.
In the above example, pluralities of piezoelectric transformers and control circuits are arranged in the high-voltage power supply unit, and a plurality of bias voltages are output to form images. Especially a high-voltage power supply unit mounted in a color image forming apparatus of a tandem system requires four bias output circuits for charging, development, transfer, and the like in correspondence with formation of cyan, magenta, yellow, and black images. The circuits corresponding to cyan (C), magenta (M), yellow (Y), and black (K) colors are controlled at almost the same bias output voltages. Piezoelectric transformers mounted in the high-voltage power supply unit are driven at almost the same frequencies (close frequencies) in the respective bias output circuits (C, M, Y, and K) for charging, development, transfer, and the like.
In this manner, a plurality of piezoelectric transformers are driven at close frequencies to output the same bias voltages. In this case, adjacent piezoelectric transformers interfere with each other via the power supply line or depending on electrostatic capacitive coupling or the like, which makes it difficult to improve the output precision of a high bias voltage. Alternatively, the image quality may degrade due to, e.g., generation of fluctuations of a high bias voltage by the interference frequency.
In order to avoid the influence on an image depending on the precision of a high bias voltage, piezoelectric transformers are arranged at a large interval. In order to suppress interference via the power supply line, the pattern length is increased or the capacitance of a decoupling capacitor is increased in designing the pattern of the power supply line. However, it is difficult to take these measures by theoretical calculation. In a case where problems are solved by many experiments, it is necessary to determine concrete measures by the many experiments. This prolongs the period of product development. Even if the problems are solvable, the high-voltage power supply unit can hardly achieve downsizing and a high image quality at the same time.